Structural panels having high strength to weight ratios are necessary or at least desirable in some operating environments, such as those encountered by aircraft. Instead of using a solid plate or sheet of material, layered structures having face sheets separated by a non-solid core have been used with much success. For example, honeycomb structures, using a honeycomb core sandwiched between two face sheets covering the open ends of the honeycomb cells, have been in use for quite some time. Other high strength to weight ratio structures can be made using various types of expanded materials sandwiched between two face sheets as well.